in Table 2-1. Since 2001, H5N1 viruses have continued to circulate in mainland China with a seasonal pattern, peaking from October to March, when the mean temperature is below 20°C (Figure 2-2). The survival and viability of influenza A virus are known to increase at lower environmental temperatures (Shortridge et al., 1998). H5N1 viruses were isolated exclusively from aquatic poultry during 2000; however, from 2001 onwards they were isolated from both aquatic and terrestrial poultry, although the rate of isolation remained greatest in ducks.
The genes of the virus isolates that encode the surface antigens haemagglutinin (HA) and neuraminidase (NA) were derived from the Goose/Guangdong/1/96 (Gs/Gd)-like lineage. Six genes, which encode internal viral proteins, arose from many other sources through reassortment and served as the basis for assignment to different genotypes (Figure 2-3). By 2001, six H5N1 reassortants (genotypes A, B, C, D, E, and X0) had been isolated from aquatic and, for the first time since 1997, terrestrial poultry (Guan et al., 1999, 2002). From 2002 onwards, eight new H5N1